While kissing is a natural behavior, you probably don't want your child making out with all their friends. Like most learned behaviors, kissing has its time and place. Sexual behaviors start when children are infants and by the time they are toddlers or preschoolers, they are acting them out. This is natural, but you can redirect them so they know how to show a moderate amount of affection as necessary.
Redirection
Different cultures have different ways to greet or kiss each other. For some families, kissing on the lips is appropriate, others prefer a kiss on each cheek. If you don't fit into these categories, you'll probably want to teach your child to kiss her friends on the cheek. This is appropriate with many cultures and will give them the freedom to kiss without making it taboo. If you don't want your child kissing anyone, they can also give a hug. Once you know the desired behavior, you can go about modifying it. If your child is kissing a specific child, you should talk to the other parents and see how they want to address it.
Two- to 3-Year-Olds
This age group is just starting to explore their bodies. Most of the behavior will involve touching private parts such as their mother's breasts, removing their clothes and fondling themselves. They will also kiss and it will usually be on the lips. This behavior is mimicked from what they see their parents and family members doing. At this age, you can encourage your child to kiss on the cheek by saying that is how you greet friends. Demonstrating this will help reinforce it.
Four- to 6-Year-Olds
These older preschoolers will mimic actions they have seen either in-person or on TV to a greater degree than younger toddlers. They might give quick kisses on the lips, longer kisses or even "play doctor" with their peers. Don't overreact. This is a natural part of exploring sexual identity. Instead, encourage your child to kiss on the cheek by saying "We kiss our friends on the cheek." Redirect the behavior before it starts by giving your child instructions such as "There is your friend. Say hi and kiss her on the cheek."
Warning Signs
Children that are sexually abused will show warning signs through behavior that is aggressively sexual. While most children kiss to some degree, a child that is being abused will show more knowledge about kissing than a child her age should have. For example, she might say that a certain adult kisses with his tongue. Other warning signs include mimicking sex acts, compulsive masturbation or trying to touch the genitalia of adults.